/*
* Copyright (c) 2008, Atmel Corporation
*
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* - Redistributions of source code must retain the above copyright notice,
* this list of conditions and the disclaimer below.
*
* Atmel's name may not be used to endorse or promote products derived from
* this software without specific prior written permission.
*
* DISCLAIMER: THIS SOFTWARE IS PROVIDED BY ATMEL "AS IS" AND ANY EXPRESS OR
* IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT ARE
* DISCLAIMED. IN NO EVENT SHALL ATMEL BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
* LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA,
* OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
* LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
* NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
* EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
/**
* Calculates the 22-bit hamming code for a 256-bytes block of data.
* \param data Data buffer to calculate code for.
* \param code Pointer to a buffer where the code should be stored.
*/
void
hamming_compute_256(const uint8_t *data, uint8_t *code)
{
unsigned int i;
uint8_t column_sum = 0;
uint8_t even_line_code = 0;
uint8_t odd_line_code = 0;
uint8_t even_column_code = 0;
uint8_t odd_column_code = 0;
/*-
* Xor all bytes together to get the column sum;
* At the same time, calculate the even and odd line codes
*/
for (i = 0; i < 256; i++) {
column_sum ^= data[i];
/*-
* If the xor sum of the byte is 0, then this byte has no
* incidence on the computed code; so check if the sum is 1.
*/
if ((popcount(data[i]) & 1) == 1) {
/*-
* Parity groups are formed by forcing a particular
* index bit to 0 (even) or 1 (odd).
* Example on one byte:
*
* bits (dec) 7 6 5 4 3 2 1 0
* (bin) 111 110 101 100 011 010 001 000
* '---'---'---'----------.
* |
* groups P4' ooooooooooooooo eeeeeeeeeeeeeee P4 |
* P2' ooooooo eeeeeee ooooooo eeeeeee P2 |
* P1' ooo eee ooo eee ooo eee ooo eee P1 |
* |
* We can see that: |
* - P4 -> bit 2 of index is 0 --------------------'
* - P4' -> bit 2 of index is 1.
* - P2 -> bit 1 of index if 0.
* - etc...
* We deduce that a bit position has an impact on all
* even Px if the log2(x)nth bit of its index is 0
* ex: log2(4) = 2,
* bit2 of the index must be 0 (-> 0 1 2 3)
* and on all odd Px' if the log2(x)nth bit
* of its index is 1
* ex: log2(2) = 1,
* bit1 of the index must be 1 (-> 0 1 4 5)
*
* As such, we calculate all the possible Px and Px'
* values at the same time in two variables,
* even_line_code and odd_line_code, such as
* even_line_code bits: P128 P64 P32
* P16 P8 P4 P2 P1
* odd_line_code bits: P128' P64' P32' P16'
* P8' P4' P2' P1'
*/
even_line_code ^= (255 - i);
odd_line_code ^= i;
}
}
/*-
* At this point, we have the line parities, and the column sum.
* First, We must calculate the parity group values on the column sum.
*/
for (i = 0; i < 8; i++) {
if (column_sum & 1) {
even_column_code ^= (7 - i);
odd_column_code ^= i;
}
column_sum >>= 1;
}
/*-
* Now, we must interleave the parity values,
* to obtain the following layout:
* Code[0] = Line1
* Code[1] = Line2
* Code[2] = Column
* Line = Px' Px P(x-1)- P(x-1) ...
* Column = P4' P4 P2' P2 P1' P1 PadBit PadBit
*/
code[0] = 0;
code[1] = 0;
code[2] = 0;
for (i = 0; i < 4; i++) {
code[0] <<= 2;
code[1] <<= 2;
code[2] <<= 2;
/**
* Verifies and corrects a 256-bytes block of data using the given 22-bits
* hamming code.
* Returns 0 if there is no error, otherwise returns a HAMMING_ERROR code.
* param data Data buffer to check.
* \param original_code Hamming code to use for verifying the data.
*/
uint8_t
hamming_correct_256(uint8_t *data, const uint8_t *original_code,
const uint8_t *computed_code)
{
/* Calculate new code */
/* we allocate 4 bytes so we can use popcount32 in one step */
uint8_t correction_code[4];
/* this byte should remain zero all the time */
correction_code[3] = 0;
/* If all bytes are 0, there is no error */
if (*(uint32_t *)correction_code == 0) {
return 0;
}
/* If there is a single bit error, there are 11 bits set to 1 */
if (popcount32(*(uint32_t *)correction_code) == 11) {
/* Get byte and bit indexes */
uint8_t byte = correction_code[0] & 0x80;
byte |= (correction_code[0] << 1) & 0x40;
byte |= (correction_code[0] << 2) & 0x20;
byte |= (correction_code[0] << 3) & 0x10;
uint8_t bit = (correction_code[2] >> 5) & 0x04;
bit |= (correction_code[2] >> 4) & 0x02;
bit |= (correction_code[2] >> 3) & 0x01;
/* Correct bit */
data[byte] ^= (1 << bit);
return HAMMING_ERROR_SINGLEBIT;
}
/* Check if ECC has been corrupted */
if (popcount32(*(uint32_t *)correction_code) == 1) {
return HAMMING_ERROR_ECC;
} else {
/* Otherwise, this is a multi-bit error */
return HAMMING_ERROR_MULTIPLEBITS;
}
}
